The invention pertains to a pneumatically powered brake system for medium and heavy vehicles. These vehicles invariably use internally expandable brake shoes lined with friction material. The braking effect in the vehicles is produced when these shoes make contact with a drum mounted on the driving or trailing wheels. The expansion of the brake shoes is done by S-profiled cams, which push the shoes out when the cam is rotated. The cam is rotated by a lever arm, at the end of which a force is applied by an air chamber, which gains its thrusting force by compressed air. These brake systems are therefore referred to as pneumatically operated brakes.
As the vehicles run, the brake is applied several times, which causes wear of the friction lining of the brake shoes along with that of brake drum. Over time, the clearance between the brake shoes and drum increases due to this wear. The enlarged clearance which is hereto called “Slack” demands more cam rotation, which in turn needs a larger stroke of the air chamber. The air chamber stroke is limited. A situation may arise when the full air chamber stroke would fail to expand the brake shoes enough for effective braking.
When the braking system of a vehicle develops slack, it needs to be adjusted for effective braking. For carrying out this operation, the vehicle is to be grounded and adjustment is to be made by trained mechanics or auto-work shops. This results in productive time loss. The slack adjuster is provided with a Worm and Gear system in such a way that by rotating the Worms the s-cam also rotates and subsequently eliminates the slack, i.e. the clearance developed between the brake shoes and the drum. The brake thus becomes effective again. This is normally termed as “brake adjustments”.
Vehicles need to be taken to a workshop for carrying out this brake adjustment which causes a loss of productive time of the vehicle. Any negligence in getting the adjustment done in time may also cause accidents due to non-effective braking. The brake adjustments are required several times during the life of the brake linings, resulting in loss of several productive days. The effectiveness of braking may also get impaired if timely adjustments are not carried out.
To circumvent the problem of manual adjustment of brakes periodically, a few manufacturers of Slack Brake Adjusters have incorporated a system to automatically adjust the slack or the clearance between the brake shoes and the drum. These systems use several parts like small gears, racks, coil springs, clutches and retainers, etc.
Patent number GB1146965, shown in FIG. 1, describes sensing from external anchoring at the vehicle chassis. In the prior art application, use of clutches, gear trains, and both secondary and primary worms results in slack adjustment.
In patent number U.S. Pat. No. 5,350,043, shown in FIGS. 2A–2C, an automatic slack adjuster causing reduction of slack present in a vehicle brake system upon brake release is defined. The slack adjuster incorporates a member cooperating with a slack adjusting rotor to determine a reference position for the rotor and to return the rotor to such reference position at the completion of each brake operational cycle. Slack adjuster 10 includes an elongated housing 20 having a bore opening adjacent one end for receiving pivot pin 16 and a bore opening adjacent an opposite end for rotatably supporting a worm gear 22. The worm gear 22 is keyed to cam shaft 18, as by a spline connection 24, for rotation about a first axis 26. Slack adjuster 10 is also connected to clevis 14 via a link 28, which is slidably supported by housing 20 and has a protruding end pivotally connected to the clevis by a pivot pin 30. Such a slack adjustment is incorporated with sensing by use of the Clevis, rotation of worm by heavy spring loaded clutch and rotor, and use of the internal worm mounted cam, pushes and spring loaded stopper. The system as described in the prior art is extremely complex and makes the system cumbersome. The complexity of the system makes the prior art system extremely difficult to implement with reduced efficiency of the system as a whole.
In publication number WO03083322, shown in FIG. 3, an automatic brake adjuster for adjusting the slack between the brake lining and brake drum of a vehicular braking system is defined. The brake adjuster includes a housing (1) accommodating an internally splined worm wheel (3) for receiving the S-cam shaft of the system; a worm shaft (4) positioned perpendicularly to the axis of the worm wheel (3) and meshing therewith; a control arrangement (12, 13, 14, 15 and 17) for controlling the movement of a clutch worm wheel (6), depending upon the angular movement of the adjuster, clutch worm wheel (6) being mounted on the worm shaft (4) and forming a clutch by serration (6′) of the clutch worm wheel engaging with the serration (4′) of the worm shaft, under the influence of a heavy compression spring (9), the control gear (12) located in the worm wheel bore of the body (1) being permanently fastened to the control arm (13) having a fixing arrangement (13′) for rigidly fixing to the chassis of the vehicle, to establish a reference; a pinion assembly comprising a ratchet (16′) and pawl (26) mechanism between the pinion (16) and the control worm screw (19), Guide (20), screw (21), spindle (23) and spring (24) and positioned parallel to the axis to control gear wheel (12) and perpendicular to the axis of worm shaft (4), the pinion (16) meshing with the control gear wheel (12) and being guided in the body (1), whereby the ratchet (16′) and pawl (26) mechanism provided between pinion (16) and control worm screw (19) functions as one-way lock and allows free rotation of pinion (16) during counter clockwise rotation under resistance from the pawl spring (27) to prevent over-riding while clutch worm gear (6) is in disengaged condition the ratchet (16′) and pawl (26) mechanism also permitting required axial movement of the control worm screw (19) to control the desired slack, the excess lining clearance being thus sensed to reference point and adjusted by said ratchet (16′) and pawl (26) mechanism to maintain optimum brake lining clearance. Hence, the prior art discloses sensing from external anchoring at chassis. Worm mounted spring to loaded clutch connected to the secondary worm and gear to provide rotation to the primary worm.
Such prior devices had certain disadvantages:                Higher cost        Too bulky and need either modification for installation or need external anchoring bracket for connection to a chassis.        Use too many parts and miniature clutch systems which tend the wear out and malfunction.        Cannot be sealed properly against ingress of water/water mists and dust, thereby exposing the parts to wear and failure.        Some devices are not interchangeable for left and right side application and the direction of thrust.        